Ignition signals generated by an automotive engine can interfere with or disturb automotive radio receiver operation. In the automotive receiver after conversion of the incoming signals to the intermediate-frequency (IF) signal, ignition signals result in pulse-type noise. This noise is superimposed on the stereo multiplex signal as, mostly oppositely phased, spikes. The elimination of this noise by conventional low-pass filters is unsatisfactory. Low-pass filtering will stretch the pulses to cause interference in the audible range after the stereo decoding process.
In an article appearing in "Funkschau", No. 25, 1984, pages 57 and 58, an active noise blanker is shown which serves to suppress noise in automotive radio receivers. It includes a circuit which detects such noise pulses caused by the automotive ignition system. By means of an electronic switch in the signal path which opens the signal path during the noise pulse, the disturbed signal is replaced by a stored signal, e.g., from a sample-and-hold circuit. In order that the blanking takes place before the disturbed signal section, the stereo multiplex signal is predelayed by 3.times.10.sup.-6 seconds with respect to the error detection signal.
In German Patent DE-A-30 28 334, issued on Feb. 11, 1982 and corresponding to U.S. Pat. No. 4,501,014, issued on Feb. 19, 1985, there is disclosed an active noise suppression circuit for briefly disturbed audio signals in which the disturbed signal portion is replaced by a suitable transfer function. The latter is adapted to the undisturbed signal by interpolation. The circuit processes digitized sample values.
European Patent Application 185 692 dated May 3, 1989, discloses a method of eliminating noise in a digitized audio signal. In the case of periodic or non-periodic signals, in one embodiment, the disturbed, short-time signal sections are replaced by a shorter signal section which is removed from the original waveform. In another embodiment there is inserted an adjacent signal section of the same length, with the "joints" being at the zero crossings of the undisturbed signal. The signal sequence is stored or delayed with the aid of a latch using the digitized sequence of sample values. The signal insertion thus corresponds to a short-time signal repetition.
The object of the invention as claimed is to provide an improved method and apparatus for noise suppression in stereo multiplex signals which permits the latter to be decoded in the presence of noise, such as the noise caused by pulses from an automotive ignition system.
The above-mentioned noise suppression techniques relate to audio signals transmitted at radio frequencies, which may also include frequency-modulated signals of the VHF range. The specific problems during stereo decoding which are due to noise-pulse stretching are not addressed. The duration of the disturbed signal section, approximately 5.times.10.sup.-6 seconds, is of the order of the period of the stereo subcarrier, approximately 26.times.10.sup.6 seconds. This refers to a stereo multiplex signal according to the FCC (=Federal Communications Commission) or EBU (=European Broadcasting Union) stereo standard, which is also referred to as "pilot tone method". Noise in the region of the subcarrier has a very disadvantageous effect on the decoding process, and it is an object of the invention to substantially reduce the effect of such noise.